Fix cython build in charmcraft build

[samuelallan72]

libpython3-dev is required to build cython.
See canonical/layer-basic#214

This problem is only evident now since we switched to charm 3.x in charmcraft builds in #115

ref. https://review.opendev.org/c/openstack/charm-keystone-ldap/+/880084

[samuelallan72]

This should fix this error from the builders:

::             creating build/temp.linux-x86_64-cpython-38/tmp/pip-install-w6m3c7en/cython_085091ebd754430b946e6159dc3c61cb/Cython/Plex
::             x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/tmp/tmpno_35jwp/include -I/usr/include/python3.8 -c /tmp/pip-install-w6m3c7en/cython_085091ebd754430b946e6159dc3c61cb/Cython/Plex/Scanners.c -o build/temp.linux-x86_64-cpython-38/tmp/pip-install-w6m3c7en/cython_085091ebd754430b946e6159dc3c61cb/Cython/Plex/Scanners.o
::             /tmp/pip-install-w6m3c7en/cython_085091ebd754430b946e6159dc3c61cb/Cython/Plex/Scanners.c:21:10: fatal error: Python.h: No such file or directory
::                21 | #include "Python.h"
::                   |          ^~~~~~~~~~
::             compilation terminated.
::             error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1
::             [end of output]

Note: I don't know why it built successfully for me locally but failed on the launchpad builders. I was using charmcraft 2.6 or 2.7.1 (charmcraft snap updated through the day and I'm not sure when).

[samuelallan72]

Thanks for the reviews! Merging without CI passing, because there are no tests yet.